The present invention relates to an improved focusing mirror for use in chemical analytical instruments, and, more particularly, to a non-planar focusing mirror. The present invention also relates to a chemical analytical instrument, utilizing a non-planar focusing mirror.
The standard method of focusing X-rays on a target is by collimation; however, this method is inherently inefficient. More recently, planar mirrors and toroidally shaped crystal structures have been developed to focus X-rays onto a sample. The latter type apparatus is disclosed, for example, in U.S. Pat. No. 4,599,741, entitled, "System For Local X-ray Excitation By Monochromatic X-rays", issued to David B. Wittry, on July 8, 1986, and is directed to the field of X-ray fluorescence analysis which is one method by which chemical microanalysis is currently being carried out.
Grazing (glancing) X-ray optics are used to reflect or focus X-rays in very large applications, but this technology cannot be scaled down to a size useful for chemical microanalysis.
Another technique utilized comprises the use of electron column instruments wherein a beam of high energy, greater than 10 keV(kiloelectron volts), electrons are focused onto a sample. Electrons interact with the sample at the point of beam impingement, causing the sample to become ionized and produce a number of measurable signals, including characteristics X-rays. The energy of the characteristic X-ray is directly related to the energy levels in the atoms, and since each atom or element of which the sample is constructed has a unique electron structure, each atom or element will emit a unique characteristic X-ray spectrum. Interpretation of the spectrum permits qualitative analysis of an unknown specimen and with proper correction procedures, the spectrum can be used to determine the composition of the specimen quantitatively.
The X-ray analysis of light elements, for example, boron(B), carbon(C), nitrogen(N) and oxygen(O) having characteristic X-ray wavelengths of 6.67 nm, 4.44 nm, 3.16 nm and 2.36 nm, respectively, has been limited by relatively low fluorescence yield in electron beam instruments such as scanning electron microscopes, electron microprobes and analytical electron microscopes. The efficiency of x-ray production is a function of the energy of incident electrons. The efficiency of x-ray detection is a function of the analytical chamber geometry, which can restrict the solid angle within which the signal is collected, and, additionally, a function of the number of incident electrons and of the x-ray absorption in the solid-state or proportional counter x-ray detectors typically used. The low fluorescence yield is a function of the physics of electron beam/solid interactions and cannot be changed. Although low atomic number elements are easily ionized by incident electrons, they are not efficient producers of characteristic X-rays. Recent improvements have been made for reducing X-ray absorption in the detector, using windowless and ultra thin window detectors so that many of the X-rays which do in fact enter the detector are counted.
A larger detector is not a practical solution to increasing the count of X-rays. The physics of X-ray detection limit the size of these detectors. In addition, the high cost of each detector precludes the use of multiple detectors.
A typical attempt to increase the solid angle of collection of the fluorescent X-rays is to position the detector close to the point of X-ray generation. An increased benefit could be obtained if a mirror could be used to collect a weak signal from a large solid angle and focus it onto the detector. The same concept could be used to collect X-rays from higher atomic numbers which exhibit shorter characteristic X-ray wavelengths; however, poor counting statistics are usually a more severe problem for light elements.
X-ray mirrors have recently been developed and comprise planar mirrors produced by depositing alternating layers of high atomic number, for example, tungsten(W), and low atomic number, for example, carbon(C), materials on a planar substrate or other means of support.
In such a structure, the inner faces are very smooth and the interface spacings are an integral multiple of the reflected X-ray wavelength. A small percentage of the incident radiation will be reflected at each interface and since the spacing is a multiple of the integral wavelength, the reflected radiation will constructively interfere, giving rise to large total reflectivities. The efficiency of reflection is a function of the materials used to build the multilayers and of the multilayer spacing.
Such mirrors must have a sufficient number of layers to reflect a sufficient fraction of the incident X-rays, but not more layers than the number needed for total reflection, as limited by absorption of X-rays in the mirror. Calculation of these numbers of layers is known in the art.
For obtaining optimal efficiency, a separate mirror would be required for each specific wavelength of interest, although a mirror optimized for a given wavelength may reflect X-rays of differing wavelengths with suitable efficiency.